Referring to FIGS. 1 and 2, an extendable worktable 1 for a machine tool is known for supporting a workpiece. The extendable worktable 1 comprises a main table 101, two side extensions 102 respectively mounted at two opposite sides of the main table 101 and movable relative to each other, two pairs of guide rods 103 respectively extended from the side extensions 102 and respectively slidably coupled to the bottom side of the main table 101, two pairs of clamping devices 104 mounted at the bottom side of the main table 101 and respectively attached to the guide rods 103, and two pairs of operating members 105 respectively eccentrically connected to the clamping devices 104 and operable to move the respective clamping devices 104 between a locking position to lock the respective guide rods 103 or an unlocking position to release the respective guide rods 103. When the clamping devices 104 are unlocked from the respective guide rods 103, the side extensions 102 are movable relative to the main table 101 to adjust the overall length of the extendable worktable 1 for holding different sizes of workpieces.
However, when wishing to move the side extensions 102 relative to the main table 101, the operator must operate the operating members 105 to unlock the respective clamping devices 104 from the respective guide rods 103. After adjustment of the position of each side extension 102, the operator must operate the operating members 105 to lock the respective clamping devices 104 to the respective guide rods 103 again. This operation procedure is complicated and inconvenient.